Do you have a vaporizer? My brother had this when he has about three and my parents used a vaporizer every night until he seemed to get over it a few months later. The vaporosor warms the air and adds some humidity which helps.

Do you have a history of asthma in the family? It's very difficult to diagnose in small children, but your gp should be able to give you an idea if he thinks its a probability (based on family history and current symptoms). My dh has asthma and I have had both kids checked when I go to the doctor and they have given me the all clear each time.

I would also recommend following the steps for eliminating dust mites and other allergens in your home, such as pet hair since they are very common triggers: air the bed each day by pulling dooms off and draping over a chair, keeping heating down as much as possible with plenty of fresh air, and vaccuum often.

Did you just get a new pet recently? Can you link it to that? My mum has a cat allergy that hasn't diagnosed until adulthood, and it's main symptom was a constant dripping nose.

Hope you find some answers.